mark kibort

So, thats a real world test that would tell you something? And, if the 10ft difference was measured, that would be a " world of difference"? Based on what information Larry? Im going to go 80mph and coast 1 mile, thats 5280 feet and if I do it again with the other tires, and stop at 5270 feet, thats a "world of differnce"? What about steering input along the way to keep the car in lane? Each little steering movement is like 1/16" toe in, and that could be the problem as some suggested.

Put a spring scale on the back bumper and pull the car accross the driveway. then put on the smaller tires. pull again. tell me what the force was to move the car forward and backward. That will be a better emperical test.

Look, I'm trying to solve the puzzle as well as the owner. However, we need to see the data , know what the computer and aquisition system uses to determine speed and go from there. However, one thing is plain as day, the heavier tire can not cause a 5mph difference in speed down a straight from a given exit speed of a turn on a road course. Did toe change? head winds?
engine clitch? driver inputs? engine temp/Power/etc? etc etc etc...........

ew928

So, is the car faster with the 17" or the 18" wheels/tires?

Is wider better?

mark kibort

I saw you last paragraph. what inaccuracy do you find there?

Yes, the planets can all be predicted in position any time of the year and be thought to be orbiting the earth as Aristotle once theorized.

Yes, airline pilots routinely pilot jets with passengers after only using the simulator.

To the point of the subject matter here, my experience with consistant lap times and changing tires that weigh from 2-5 lbs more as well as differnet diameters as much as 1" different, has not yeilded any statistical changes in lap times overall. We are talking 3 tracks, 120 race days , consistant narrow band weather temps and conditions, and a car that has not changed in performance that much.

mk

mark kibort

my 17s and 18s were identical. even with heavier tires on the 18s. I did like the taller sidewalls for a while with the 17s for a while, until i found the right pressures to make them work the same. weight of the same tire on the different sized rim was near identical too.

mk

Larry Herman

Mark, you are clearly off the deep end on every discussion that might be of any technical nature where you can dazzle us with your math. All I can think about when I read your posts is that physics predicted that no car could go faster than 150 mph average at Indy or do the quarter mile is less than, I forget - 7 seconds or something, because all of the calculations were flawed. You seem to be acting like the kid who lacked attention and so made up for it by having to be right all of the time. I have tried to give you the benefit of the doubt but frankly I am tired of dealing with you.

Take for example your inane come back to my "10 feet could be all the difference in the world". I did say could be, not absolutely was. Your coast down example of 5280 feet is a ridiculous postulation to try and prove me wrong. I personally do not see where you are helping the situation, just ram-rodding your own agenda past everyone until we tire of arguing.

Tell me again how you are "trying to solve the puzzle" with your approach? On second though, don't, because I no longer give a damn.

Brinkley

Sorry for the absence in this conversation. I wish I could save a screen shot of the Motec data. If someone knows how to save a screen shot then attached to Rennlist let me know.

A couple of answers to some of the questions.
Toe in .8mm
I don't know rolling friction and I didn't do a roll down test.

MPH at exit 17's 62 18's 61
Top speed 121 119 time difference .26 sec on one straight. another example on a shorter straight is another .10 of a sec.
Speed is generated from both GPS and ground speed, both match up pretty well.

Another good test is to run the 17's with larger 295's and see if I get the "faster" acceleration and the "faster" corning speeds. a compromise. small contact patch vs. 315 but larger than current 275's.

jgrant

Wow. Never thought I'd see a thread that made me long for the good old days of HP/Torque.

M758

Mark,
Like it or not I KNOW when I have lots of rubber on the tires and when I don't. When I have less rubber the car more responsive. That is combination of weight and the smaller diameter on the worn tire. Strangly I don't notice a big difference on fuel weight, but I can sense the tire weight. Fresh tires feel like heavy shoes when old worn tires feel like light weight track shoes.

This is the best I can describe it.

Brinkley

Regarding the question which is faster, well......

At sebring I was able to turn a 2:25 with both, however when you look at the data traces, you see the wider tires make the time in the braking and cornering while the thinner 17's made the time on the straights.

At Homestead, I did a 1:37 on 17's but was only able to do a 1:38.4 on the 18's. The data showed same thing, faster out of the corners and slightly slower through the center.

Regarding, shift points, you could put the lap data on top of each other and its as close as a club race can be. I'm shifting inside 100 rpm on each straight. 6843 vs. 6881 for example.

mark kibort

Larry,

Dont be a jerk!

Its a discussion. Im not doing it for the attention. I hate having to go back and reiterate the obvious. Im doing to solve the puzzle

Now, I only gave one factor that can accurately predict its effect on acceleration. It can be questioned, but no on here so far has contested it. again, Im just the messenger here of Newton. as far as the other factors, HECK, i believe that he saw 5mph differnce. I want to know why as much as he does. I like these kind of puzzles AND if we find out why the data or calculatinos are flawed, then we all learn something.

As far as predicting indy cars and quartermile times, and downforce, etc, those were FLAWED predictions because their physics were obviously flawed. the equations are as old as the modern age itself. Newton hasnt been proven wrong to date, but many mis use his equations. Garbage in garbage out, right?

I didnt put the "1 mile coast down remark" to try and prove you wrong, i did it to show that there would be even MORE factors that could skew the outcome.

I just thought of something. He could do a coast down, but only do it from 60-mph to 40 mph on the same stretch of hyway. This would be good so that aero wasnt that much of a factor AND i could get a HP/seconds value of what it took to slow the car down. If the time was even fractionally different, it could represent 5-10 hp. we also could see the total power of the aero and rolling friction and postulate what a small change in it might do to rolling friction forces. For example, 40-60 in about 1.5 seconds requires somewhere around 300hp. find some flat pavement and do the test in both directions, and get some time value and we can attach a hp value to coast down. Its things like this that can be valuable in predicting or finding some partial answers.

However, i still would like to see how the data aqu. system works.

Hey, by the way, Im going to laguna for a dual race weekend. Im going to run one race on the toyos and another race on the same sized 4 lb lighter hoosiers.
I can use my I-phone for some G readings, at least it is good for max values as the graphs are not that useful. Hey, let me know and Ill do some testing this weekend at the track.

mark kibort

You are probably talking full tread vs near bald or shaved levels. Yes, ive driven full tread toyos and they feel real heavy, but they are not so much heavy as they are squirmy. (tread movment) shaved toyos feel like slicks (albeit slidier than hoosiers) and are firm, but the weight is not a factor, unless you can tell 1hp rate difference which is nearly impossible. Could the difference you feel due to full tread action??

ew928

So on tracks with longer straight sections and less dependence on outright cornering prowess, you can run the 17" setup. And save the 18" tires for the cornering tracks.

mark kibort

Do you have to input tire diameter? your toe is almost 0, (1/32" ) so that wont be a factor. you have a 1mph difference on the exit of the turn in question. You say, its GPS and ground speed? How does it measure ground speed?
2mph at top speed difference and you are losing .25 seconds total on that straight. even the fact that the 315s are taller, wouldnt make that much of a difference in total time , as you would have trade offs in the gear shifts to counteract the "effective" 1% gearing differences.

To me, its all coming back to the conditions. aero possibly. what about the engine performance. Certainly, gas or other factors could be an issue.
head wind? wing settings? Same weight and amount of gas in the tank?
sounded like one run was earlier in the day and the other was when it was hot. track temp, and outside tempurature can result in 1-2mph differences sometimes. Again, we have to rule out the weight of the tire as its effects are on par with less than 2 hp. the aero drag of the slightly larger face showing below the spliter and the 1.5" wider rear tires showing below the bodywork could be some of it. Both Hossiers, so i woulnt think the stick would make that much difference as far as rolling friction changes.

I think you need to go back with the same set up and try again. Eventually, go back to the 275s and 245s and see if you can find the improvments.

puzzeling.

Personally, I go back and forth between 5 lb heavier toyo front 305x35s and 275x35s. the heavier tire is felt in the steering, and the braking and cornering is improved. I felt no difference in the acceleration and top speeds seem to be near the same. car was more stable and more manuverable in traffic. since im doing more of a dramatic change and my top speeds are near yours with the same size front tires, I should be able to get some kind of data this weekend.
I might even buy that new program that turns the I phone into a data aquasition system.

mark kibort

or play with pressures. what pressures are you running for both sets? I found that less pressure in the 18s helped run the tire with the same feel as the 17s that were working well. I was generally about 2psi less pressure in the tires to start.

mk

jgrant

For what it's worth, when I switch between 17's and 18's, I have to slightly adjust my suspension, as the geometry is slightly different.

Also, a smaller tire diameter will produce an effective gearing change, so will improve acceleration.

There's a reason why a lot of good autocrossers (in stock classes) look for the smallest diameter wheel/tire combo they can legally run.